First Law of Thermodynamics

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First Law of Thermodynamics

• The First Law of Thermodynamics states that
  energy can neither be created or destroyed (just
  like total mass!)

Accumulation In Out Generation
Consumption
But generation0 and consumption0 since energy
cannot be created or destroyed so the general
balance becomes
Accumulation In Out
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Energy Balances on Closed Systems

• Closed System no material crosses the system
  boundary over a period of time (e.g., batch
  process).
• General balance equation is
• Although no mass crosses the boundaries, input?0
  and output?0 since energy can be transferred
  across the boundary. Therefore, the balance
  becomes

Accumulation Input Output
1st Law of Thermodynamics for a Closed System
(? final initial)
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Notes on Energy Balances for a Closed System

• Possible Simplifications
• if Tsystem Tsurroundings, then Q 0 since no
  heat is being transferred due to temperature
  difference
• if the system is perfectly insulated, then Q 0
  (system is adiabatic) since no heat is being
  transferred between the system and the
  surroundings
• if system is not accelerating, then ?Ek 0
• if system is not rising or falling, then ?Ep 0
• if energy is not transferred across the system
  boundary by a moving part (e.g., piston,
  impeller, rotor), then W 0
• if system is at constant temperature (system is
  isothermal), no phase changes or chemical
  reactions are taking place, and only minimal
  pressure changes, then ?U 0

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Examples of Closed Systems
Example 1 Heating water in a sealed container
Example 2 Compressing a gas in a cylinder.
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Problem (Problem 7.9 FR)

• Write and simplify the closed-system energy
  balance for each of the following processes, and
  state whether nonzero heat and work terms are
  positive or negative. Begin by defining the
  system.
• The contents of a closed flask are heated from
  25?C to 80?C.
• A tray filled with water at 20?C is put into a
  freezer. The water turns into ice at -5?C.
  (Note When a substance expands it does work on
  its surroundings and when it contracts the
  surroundings do work on it.)
• A chemical reaction takes place in a closed
  adiabatic (perfectly insulated) rigid container.
  
• Repeat part (c), only suppose the reactor is
  isothermal rather than adiabatic and that when
  the reaction was carried out adiabatically the
  temperature in the reactor increased.

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Problem (Example 7.3-1)

• A gas is contained in a cylinder fitted with a
  movable piston. The initial gas temperature is
  25?C.
• The cylinder is placed in boiling water with the
  piston held in a fixed position by a clamp. Heat
  in the amount of 2.00 kcal is transferred to the
  gas, which equilibrates at 100?C (and a higher
  pressure). The piston is then released, and the
  gas does 100 J of work in moving the piston to
  its new equilibrium position. The final gas
  temperature is 100 ?C.
• Write the energy balance equation for each of the
  two stages of this process, and in each case,
  solve for the unknown energy term in the
  equation. Consider the gas in the cylinder to be
  the system, neglect the change in potential
  energy of the gas as the piston moves vertically,
  and assume the gas behaves ideally. Express all
  energies in Joules.